Optical signals can be generated via laser systems that produce coherent stimulated emission in response to electrical input driver signals. The magnitude or intensity of optical signals generated via laser systems are typically characterized by temporal variations including a ramp time the optical cavity initially achieves stability, and a tail time while the optical cavity ceases to emit optical signals following turn off of the driver signals. Laser-generated optical signals are employed in various fields including communication systems that employ optical signals conveyed via fiber optic connections to transmit embedded information. To achieve high bit rates in such communication links, it is often desired to use fast optical pulses with adjustable amplitude versus time profiles. High efficiency pulse shaping can assist in forming ultra short bit streams to be transmitted as short bursts of light allowing an increase in the data transfer rate. Pulse shaping may also play a role in ultra fast optical switching filtering and amplification.
Optical amplifiers are devices employed in optical communications to amplify an incoming optical signal and output the amplified signal. Some optical amplifiers are operated by control signals that activate or deactivate semiconductor elements within the optical amplifier to allow the semiconductor optical amplifier to provide gain to an incoming optical signal, and output the amplified signal, according to control signals.
The generation of sub nanosecond optical pulses with controllable duration is a difficult laser engineering problem. For example, passively q-switched solid-state lasers (e.g. microchip lasers) emit pulses of duration in the 500-1000 ps window, but the pulse duration is not adjustable and cannot be adjusted to a smaller duration due to limitations inherent in the pulse formation dynamics. In actively q-switched lasers the pulsewidth can be controlled to some extent, but the pulse duration is longer (typically >1 ns).
Gain-switched diode lasers generate optical pulses of arbitrary duration according to a driving electric signal. However, these lasers must be driven by peak currents close to 1 ampere to be of practical use and the generation of fast, sub-nanosecond current transients of this magnitude is an extremely challenging electronics design problem.
Semiconductor lasers can be driven in the “gain-switch spike” mode, in which the laser is barely pushed above threshold and then the emission is quickly truncated after the first relaxation oscillator. However, the gain switch spike is <100 ps long, its duration uncontrollable, and the emitted power is very low and of limited use. Mode-locking typically results in shorter (tens of picoseconds or shorter) and not adjustable pulse durations, usually at very high pulse repetition rates (˜1 MHz or greater).
Intracavity spectral filters and pulse pickers can be employed to obtain longer pulses or lower repetition rates, respectively, but such devices tend to greatly increase complexity while offering only modest pulse control. Semiconductor optical amplifiers are, in principle, better modulators: they provide optical gain rather than loss, exhibit very high and stable pulse contrast (>50 dB), and do not suffer from photorefractive damage. However, because of carrier lifetime, the shortest pulses that can be generated via semiconductor optical amplifiers are typically greater than 2 nanoseconds.